Electro-osmotic displays and materials for making the same

ABSTRACT

Disclosed herein are novel electrophoretic displays and materials useful in fabricating such displays. In particular, novel encapsulated displays are disclosed. Particles encapsulated therein are dispersed within a suspending, or electrophoretic, fluid. This fluid may be a mixture of two or more fluids or may be a single fluid. The displays may further comprise particles dispersed in a suspending fluid, wherein the particles contain a liquid. In either case, the suspending fluid may have a density or refractive index substantially matched to that of the particles dispersed therein. Finally, also disclosed herein are electro-osmotic displays. These displays comprise at least one capsule containing either a cellulosic or gel-like internal phase and a liquid phase, or containing two or more immiscible fluids. Application of electric fields to any of the electrophoretic displays described herein affects an optical property of the display.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. Ser. No. 11/426,672,filed Jun. 27, 2006 (Publication No. 2006/0245038).

U.S. Ser. No. 11/426,672 is a continuation of U.S. Ser. No. 10/329,023,filed Dec. 23, 2002 (now U.S. Pat. No. 7,109,968).

U.S. Ser. No. 10/329,023 is a continuation of U.S. Ser. No. 09/140,846,filed Aug. 27, 1998 (now U.S. Pat. No. 6,727,881).

U.S. Ser. No. 09/140,846 is, in turn, a continuation-in-part of each ofthree earlier applications, namely:

-   -   (a) U.S. Ser. No. 08/504,896, filed Jul. 20, 1995 (now U.S. Pat.        No. 6,124,851);    -   (b) U.S. Ser. No. 08/983,404 filed Mar. 26, 1999 (now U.S. Pat.        No. 7,106,296); and    -   (c) U.S. Ser. No. 08/935,800, filed Sep. 23, 1997 (now U.S. Pat.        No. 6,120,588).

U.S. Ser. No. 09/140,846 also claims benefit of each of the followingapplications:

-   -   (1) U.S. Ser. No. 60/057,133, filed Aug. 28, 1997;    -   (2) U.S. Ser. No. 60/057,716, filed Aug. 28, 1997;    -   (3) U.S. Ser. No. 60/057,799, filed Aug. 28, 1997;    -   (4) U.S. Ser. No. 60/057,163, filed Aug. 28, 1997;    -   (5) U.S. Ser. No. 60/057,122, filed Aug. 28, 1997;    -   (6) U.S. Ser. No. 60/057,798, filed Aug. 28, 1997;    -   (7) U.S. Ser. No. 60/057,118, filed Aug. 28, 1997;    -   (8) U.S. Ser. No. 60/059,543, filed Sep. 19, 1997;    -   (9) U.S. Ser. No. 60/059,358, filed Sep. 19, 1997;    -   (10) U.S. Ser. No. 60/065,630, filed Nov. 18, 1997;    -   (11) U.S. Ser. No. 60/065,605, filed Nov. 18, 1997;    -   (12) U.S. Ser. No. 60/065,629, filed Nov. 18, 1997;    -   (13) U.S. Ser. No. 60/066,147, filed Nov. 19, 1997;    -   (14) U.S. Ser. No. 60/066,245, filed Nov. 20, 1997;    -   (15) U.S. Ser. No. 60/066,246, filed Nov. 20, 1997;    -   (16) U.S. Ser. No. 60/066,115, filed Nov. 21, 1997;    -   (17) U.S. Ser. No. 60/066,334, filed Nov. 21, 1997;    -   (18) U.S. Ser. No. 60/066,418, filed Nov. 24, 1997;    -   (19) U.S. Ser. No. 60/071,371, filed Jan. 15, 1998;    -   (20) U.S. Ser. No. 60/070,940, filed Jan. 9, 1998;    -   (21) U.S. Ser. No. 60/072,390, filed Jan. 9, 1998;    -   (22) U.S. Ser. No. 60/070,939, filed Jan. 9, 1998;    -   (23) U.S. Ser. No. 60/070,935, filed Jan. 9, 1998;    -   (24) U.S. Ser. No. 60/074,454, filed Feb. 12, 1998;    -   (25) U.S. Ser. No. 60/076,955, filed Mar. 5, 1998;    -   (26) U.S. Ser. No. 60/076,959, filed Mar. 5, 1998;    -   (27) U.S. Ser. No. 60/076,957, filed Mar. 5, 1998;    -   (28) U.S. Ser. No. 60/076,956, filed Mar. 5, 1998;    -   (29) U.S. Ser. No. 60/076,978, filed Mar. 5, 1998;    -   (30) U.S. Ser. No. 60/078,363, filed Mar. 18, 1998;    -   (31) U.S. Ser. No. 60/081,374, filed Apr. 10, 1998;    -   (32) U.S. Ser. No. 60/081,362, filed Apr. 10, 1998;    -   (33) U.S. Ser. No. 60/083,252, filed Apr. 27, 1998;    -   (34) U.S. Ser. No. 60/085,096, filed May 12, 1998;    -   (35) U.S. Ser. No. 60/090,223, filed Jun. 22, 1998;    -   (36) U.S. Ser. No. 60/090,222, filed Jun. 22, 1998;    -   (37) U.S. Ser. No. 60/090,232, filed Jun. 22, 1998;    -   (38) U.S. Ser. No. 60/092,046, filed Jul. 8, 1998;    -   (39) U.S. Ser. No. 60/092,050, filed Jul. 8, 1998;    -   (40) U.S. Ser. No. 60/092,742, filed Jul. 14, 1998; and    -   (41) U.S. Ser. No. 60/093,689, filed Jul. 22, 1998.

The aforementioned U.S. Ser. No. 08/983,404 is the U.S. national stageof International Application No. PCT/US96/12000, filed Jul. 19, 1996.

The entire disclosures of all the aforementioned applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to electro-osmotic displays, particularlyencapsulated electro-osmotic displays, and to materials useful infabricating such displays.

BACKGROUND OF THE INVENTION

Electrophoretic displays have been the subject of intense research anddevelopment for a number of years. Electrophoretic displays haveattributes of good brightness and contrast, wide viewing angles, statebistability, and low power consumption when compared with liquid crystaldisplays. Nevertheless, problems with the long-term image quality ofthese displays has, to date, prevented their widespread usage.

The recent invention of encapsulated electrophoretic displays solvesmany of these problems and offers additional advantages compared toliquid crystal displays. Some added advantages are the ability to printor coat the display material on a wide variety of flexible and rigidsubstrates. The clustering and settling problems, which plagued priorart electrophoretic displays and resulted in inadequate lifetimes forthe displays are now overcome.

The purpose of this disclosure is to describe electrophoretic displays,especially encapsulated electrophoretic displays, and classes ofmaterials, as well as some specific materials, which should be useful intheir construction.

SUMMARY OF THE INVENTION

The successful construction of an encapsulated electrophoretic displayrequires the proper interaction of several different types of materialsand processes. Materials such as a polymeric binder, a capsule membrane,and the electrophoretic particles and fluid must all be chemicallycompatible. The capsule membranes may engage in useful surfaceinteractions with the electrophoretic particles, or may act as an inertphysical boundary between the fluid and the binder. Polymer binders mayset as adhesives between capsule membranes and electrode surfaces.

In some cases, a separate encapsulation step of the process is notnecessary. The electrophoretic fluid may be directly dispersed oremulsified into the binder (or a precursor to the binder material) toform what may be called a “polymer-dispersed electrophoretic display”.In such displays, the individual electrophoretic phases may be referredto as capsules or microcapsules even though no capsule membrane ispresent. Such polymer-dispersed electrophoretic displays are consideredto be subsets of encapsulated electrophoretic displays.

In an encapsulated electrophoretic display, the binder materialsurrounds the capsules and separates the two bounding electrodes. Thisbinder material must be compatible with the capsule and boundingelectrodes and must possess properties that allow for facile printing orcoating. It may also possess barrier properties for water, oxygen,ultraviolet light, the electrophoretic fluid, or other materials.Further, it may contain surfactants and cross-linking agents, whichcould aid in coating or durability. The polymer-dispersedelectrophoretic display may be of the emulsion or phase separation type.

The present invention provides electrophoretic displays, particularlyencapsulated electrophoretic displays, and materials for use in suchdisplays. The capsules may be spherical or non-spherical in shape. Inelectrophoretic displays, at least some of the particles are moved orrotated by application of electric fields. The electric field may be analternating-current field or a direct-current field. The electric fieldmay be created by at least one pair of electrodes disposed adjacent abinder material containing the particles. The particles may be absorbingpigments, scattering pigments or luminescent particles, for example. Theparticles may be made up of some combination of dye, pigment, polymer.

Such displays may also include, for example, one type of particle thatretroreflects, or substantially retroreflects, light and another typethat absorbs light. Application of an electric field may cause theparticles in an encapsulated display to orient so that the capsuleretroreflects, or substantially retroreflects, light. Application ofanother electric field may cause the particles to orient so that thecapsule absorbs, or does not retroreflect, light. A display may alsoinclude a reflective substrate, so that orientation of one type ofparticle in a particular pattern causes light to pass through thecapsule to the substrate, which reflects light. Orientation of a secondtype of particle in a particular pattern causes the capsule to absorb,or otherwise not reflect, light. Types of retroreflective and reflectivematerials that may be used in constructing a retroreflective orreflective substrates, respectively, include glass spheres anddiffractive reflecting layers.

Another type of display has particles of differing colors. Such adisplay has at least two, and preferably at least, three differentspecies of particles, with each type of particle having a differentelectrophoretic mobility. The different electrophoretic mobilitiesprovide the particles with substantially non-overlapping electrophoreticmobilities, so that application of different electric fields causesdifferent subsets of the colored particles to be viewed at the surfaceof the capsule.

Another type of display includes luminescent particles and a visiblelight-blocking medium, which may contain light-absorbing particles ordyes. Application of different electric fields may cause the particlesto luminesce selectively or uniformly at the front (eyes see a brightpixel) or rear (fluid absorbs radiation) of the capsule. Application ofdifferent electric fields may cause either the luminescent particles orthe light-blocking particles to rise to the capsule surface, resultingin either a light or a dark appearance to the capsule, respectively.

In another type of electrophoretic display, the particles may themselvesbe encapsulated pigments, dyes, pigment dispersions, dye solutions, or acombination of any of these. These particles are dispersed in asuspending fluid and are then encapsulated into capsules in a binder.The particles may be dispersed within a suspending fluid and may eachcontain a plurality of solid particles or a dye or both. The suspendingfluid can be a single fluid or a mixture of two or more fluids. In oneembodiment, the particles may have a diameter from between about 10 nmand about 5 .μm, whereas the capsules may have a diameter from betweenabout 5 .μm and about 200 .μm. In another embodiment, the particles mayhave a flexible outer surface or may be a polymeric layer surrounding adye or dye solution.

The advantage of this system is that known emulsification orencapsulation techniques can be used to make improved particles, withbetter control of absorbance, optical properties, charge, mobility,shape, size, density, surface chemistry, stability, and processibility.There are vast numbers of dyes and/or particles and liquids of allpolarities that can be used to gain a high level of control over theoptical properties of the system. It is possible to create particleswhich are capsules containing dyes and/or particles in order to obtainproperties difficult to achieve with pigments. The present inventionrelates to these encapsulated electrophoretic displays and thematerials, such as dyes, pigments, binder, etc. that may be useful intheir construction.

Encapsulated electrophoretic displays may include two or more differenttypes of particles. Such displays may include, for example, displayscontaining a plurality of anisotropic particles and a plurality ofsecond particles in a suspending fluid. Application of a first electricfield may cause the anisotropic particles to assume a specificorientation and present an optical property. Application of a secondelectric field may then cause the plurality of second particles totranslate, thereby disorienting the anisotropic particles and disturbingthe optical property. Alternatively, the orientation of the anisotropicparticles may allow easier translation of the plurality of secondparticles. The particles may have a refractive index that substantiallymatches the refractive index of the suspending fluid.

Finally, an encapsulated display may comprise an electro-osmoticdisplay. Such a display may comprise capsules containing a refractiveindex matching fluid, that moves within the capsule to create ahomogeneous capsule upon application of an electric field. The capsulemay also contain a porous internal material, such as an alkylcellulose,that swells upon movement of the refractive index matching fluid withinthe capsule. An electro-osmotic display may also include two or moreimmiscible fluids, that move within the capsule to create a differentoptical property upon application of an electric field. The opticaleffect may result from a planar index mismatch or a non-planar indexmismatch.

Materials for use in creating electrophoretic displays relate to thetypes of materials, including, but not limited to, particles, dyes,suspending fluids, and binders used in fabricating the displays. In oneembodiment, types of particles that may be used to fabricate suspendedparticle displays include scattering pigments, absorbing pigments andluminescent particles. Such particles may also be transparent. Preferredparticles include titania, which may be coated in one or two layers witha metal oxide, such as aluminum oxide or silicon oxide, for example.Such particles may also be retroreflective or have a reflective coating.Such particles may be constructed as corner cubes. Luminescent particlesmay include, for example, zinc sulfide particles. The zinc sulfideparticles may also be encapsulated with an insulative coating to reduceelectrical conduction. Light-blocking or absorbing particles mayinclude, for example, dyes or pigments.

A suspending (i.e., electrophoretic) fluid may be a high resistivityfluid. The suspending fluid may be a single fluid, or it may be amixture of two or more fluids. The suspending fluid, whether a singlefluid or a mixture of fluids, may have its density substantially matchedto that of the particles within the capsule. The suspending fluid may bea halogenated hydrocarbon, such as tetrachloroethylene, for example. Thehalogenated hydrocarbon may also be a low molecular weight polymer. Onesuch low molecular weight polymer is poly(chlorotrifluoroethylene). Thedegree of polymerization for this polymer may be from about 2 to about10.

Types of dyes for use in electrophoretic displays are commonly known inthe art. They may be soluble in the suspending fluid. These dyes mayfurther be part of a polymeric chain. Dyes may be polymerized bythermal, photochemical, and chemical diffusion processes. Single dyes ormixtures of dyes may also be used.

Furthermore, capsules may be formed in, or later dispersed in, a binder.Materials for use as binders include water-soluble polymers,water-dispersed polymers, oil-soluble polymers, thermoset polymers,thermoplastic polymers, and UV- or radiation-cured polymers.

The invention will be understood further upon consideration of thefollowing drawings, description and claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of encapsulated light scatteringparticles.

FIG. 2 shows a capsule containing particles in a suspending fluid andhaving a pair of electrodes disposed adjacent thereto.

FIG. 3 shows a capsule containing light-absorbing particles in asuspending fluid and having a reflective or retroreflective substratedisposed at the bottom face of the capsule. The particles are shownmoved toward one of the pair of electrodes so that light can passthrough the capsule and be reflected by the substrate.

FIG. 4 shows the capsule of FIG. 3 in which the particles are moved toblock light from reaching the substrate, thereby preventing light frombeing reflected by the substrate.

FIG. 5 shows a capsule containing light-absorbing particles andretroreflecting particles.

FIG. 6A shows a capsule containing a reflecting corner cube at itsbottom face and particles. In this illustration, the particles arepositioned so that light can pass through the capsule and be reflectedby the corner cube.

FIG. 6B shows a capsule containing a reflecting corner cube at itsbottom face and particles. In this illustration, the particles arepositioned so that light can pass through the capsule and be reflectedby the corner cube.

FIG. 6C shows a microcapsule containing a reflecting corner cube at itsbottom face and particles. In this illustration, the particles arepositioned so that light cannot pass through the capsule and bereflected by the corner cube.

FIG. 7 shows how a capsule may reflect light.

FIG. 8A shows a capsule of FIG. 7 in which particles contained withinthe capsule are positioned so as to allow light to enter the capsule andbe reflected.

FIG. 8B shows a capsule of FIG. 7 in which particles contained withinthe capsule are positioned so as to prevent light entering the capsulefrom being reflected.

FIG. 9 shows a capsule containing luminescent particles andlight-absorbing particles. In this illustration, the luminescentparticles are positioned toward the top face of the capsule, therebyproviding light.

FIG. 10 shows a capsule of FIG. 9 in which the light-absorbing particlesare positioned toward the top face of the capsule, thereby blockinglight from exiting the capsule.

FIG. 11 shows a capsule disposed adjacent a reflective substrate and twoelectrodes, in which the particles within the capsule are aligned so asto allow light to pass through the capsule and be reflected by thesubstrate.

FIG. 12A shows two capsules in a binder disposed adjacent a reflectivesubstrate and two electrodes, in which the particles within the capsuleare aligned so as to allow light to pass through the capsule and bereflected by the substrate.

FIG. 12B shows a capsule disposed adjacent a reflective substrate andtwo electrodes, in which the particles within the capsule are aligned soas to prevent light from passing through the capsule and being reflectedby the substrate.

FIG. 13A is an illustration of an apparatus for performingemulsion-based encapsulation.

FIG. 13B is an illustration of an oil drop of suspending fluid havingwhite and black particles dispersed within it.

FIG. 13C is an illustration of an oil drop of darkly dyed suspendingfluid having white microparticles and charge control agents dispersedwithin it.

FIG. 14 illustrates one embodiment of non-spherical capsules.

FIG. 15A illustrates, through a cross-section view, non-sphericalcapsules that are slightly flattened. FIG. 15B illustrates, through across-section view, non-spherical capsules that are heavily flattened.FIG. 15C illustrates, through a cross-section view, polyhedron capsules.

Like reference characters in the drawings represent corresponding parts.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to improved encapsulated electrophoretic displaysand materials useful in their construction. Generally, an encapsulatedelectrophoretic display includes one or more species of particle thateither absorb or scatter light. One example is a system in which thecapsules contain one or more species of electrophoretically mobileparticles dispersed in a dyed suspending fluid. Another example is asystem in which the capsules contain two separate species of particlessuspended in a clear suspending fluid, in which one species of particleabsorbs light (black), while the other species of particle scatterslight (white). There are other extensions (more than two species ofparticles, with or without a dye, etc.). The particles are commonlysolid pigments, dyed particles or pigment/polymer composites.

Electrophoretic displays of the invention are described below. Thesedisplays are preferably microencapsulated electrophoretic displays. Alsodescribed below are materials that may be useful in such displays.

I. ELECTROPHORETIC DISPLAYS

An object of the invention is to provide a highly-flexible, reflectivedisplay which can be manufactured easily, consumes little (or no in thecase of bistable displays) power, and can, therefore, be incorporatedinto a variety of applications. The invention features a printabledisplay comprising an encapsulated electrophoretic display medium. Theresulting display is flexible. Since the display media can be printed,the display itself can be made inexpensively.

An encapsulated electrophoretic display can be constructed so that theoptical state of the display is stable for some length of time. When thedisplay has two states which are stable in this manner, the display issaid to be bistable. If more than two states of the display are stable,then the display can be said to be multistable. For the purpose of thisinvention, the term bistable will be used to indicate a display in whichany optical state remains fixed once the addressing voltage is removed.The definition of a bistable state depends on the application for thedisplay. A slowly-decaying optical state can be effectively bistable ifthe optical state is substantially unchanged over the required viewingtime. For example, in a display which is updated every few minutes, adisplay image which is stable for hours or days is effectively bistablefor that application. In this invention, the term bistable alsoindicates a display with an optical state sufficiently long-lived as tobe effectively bistable for the application in mind. Alternatively, itis possible to construct encapsulated electrophoretic displays in whichthe image decays quickly once the addressing voltage to the display isremoved (i.e., the display is not bistable or multistable). As will bedescribed, in some applications it is advantageous to use anencapsulated electrophoretic display which is not bistable. Whether ornot an encapsulated electrophoretic display is bistable, and its degreeof bistability, can be controlled through appropriate chemicalmodification of the electrophoretic particles, the suspending fluid, thecapsule, and binder materials.

An encapsulated electrophoretic display may take many forms. The displaymay comprise capsules dispersed in a binder. The capsules may be of anysize or shape. The capsules may, for example, be spherical and may havediameters in the millimeter range or the micron range, but is preferablyfrom ten to a few hundred microns. The capsules may be formed by anencapsulation technique, as described below. Particles may beencapsulated in the capsules. The particles may be two or more differenttypes of particles. The particles may be colored, luminescent,light-absorbing or transparent, for example. The particles may includeneat pigments, dyed (laked) pigments or pigment/polymer composites, forexample. The display may further comprise a suspending fluid in whichthe particles are dispersed.

In electrophoretic displays, the particles may be oriented or translatedby placing an electric field across the capsule. The electric field mayinclude an alternating-current field or a direct-current field. Theelectric field may be provided by at least one pair of electrodesdisposed adjacent to a display comprising the capsule.

Throughout the specification, reference will be made to printing orprinted. As used throughout the specification, printing is intended toinclude all forms of printing and coating, including: premeteredcoatings such as patch die coating, slot or extrusion coating, slide orcascade coating, and curtain coating; roll coating such as knife overroll coating, forward and reverse roll coating; gravure coating; dipcoating; spray coating; meniscus coating; spin coating; brush coating;air knife coating; silk screen printing processes; electrostaticprinting processes; thermal printing processes; and other similartechniques. A “printed element” refers to an element formed using anyone of the above techniques.

FIG. 1 shows an electrophoretic display of the invention. The binder 11includes at least one capsule 13, which is filled with a plurality ofparticles 15 and a dyed suspending fluid 17. In one embodiment, theparticles 15 are titania particles. When a direct-current electric fieldof the appropriate polarity is applied across the capsule 13, theparticles 15 move to the viewed surface of the display and scatterlight. When the applied electric field is reversed, the particles 15move to the rear surface of the display and the viewed surface of thedisplay then appears dark.

FIG. 2 shows an electrophoretic display of the invention. This displaycomprises anisotropic particles 10 and a second set of particles 12 in acapsule 14. The capsule has electrodes 16 and 16′ disposed adjacent it.The electrodes are connected to a source of voltage 18, which mayprovide an alternating-current (AC) field or a direct-current (DC) fieldto the capsule 14. In this display, the anisotropic particles 10 areoriented by an AC field so as to allow light to pass through thecapsule. Brownian motion normally slowly restores the particles to anisotropic state. In this display, a clear index of refraction matchedsecond set of particles 12 is, however, used to provide internalturbulence and disorient the anisotropic particles. Applying a DC fieldthat is switched back and forth at a much lower frequency causes thesecond set of particles to translate and disturb any orientedanisotropic particles. This will cause the display to reset to itsscattering state much more quickly. The display cell appears dark upondisorientation of the anisotropic particles. This scheme will work in anencapsulated, polymer dispersed, or normal liquid cell.

In another embodiment of the invention, an electrophoretic display thatuses a retroreflecting surface is described. This implementation neednot be encapsulated, but rather may also be embodied in the form of astandard electrophoretic display. FIGS. 3 and 4 show such a display.

In FIG. 3, capsule 20 is filled with a suspending fluid, which may be afluid of high resistivity and particles 22. When the particles areattracted towards electrode 24 by the application of an electric field,the particles take up a minority of the viewable area of the display.This exposes clear electrode 26 and allows the light to reflect off thesurface 28. This surface may be composed of glass spheres, a diffractivereflecting layer, such as a holographically formed reflector, forexample, any other known retroflecting surface, or any other surfacewhich contrasts with the particles. The capsule then has the appearanceof substrate 28.

In FIG. 4, the second state of the capsule is displayed. Particles 22contained within capsule 20 migrate towards electrode 26 by theapplication of an electric field. These particles thus obscure surface28, and the capsule, when viewed from the top, then appears to have theproperties of the particle.

FIG. 5 shows another embodiment of the invention. In this embodiment, areflective display may be made selectively retroreflective bymanipulating charged particles to either block a retroreflective lightpath or create a retroreflective surface. In this embodiment, capsule 30contains retroreflecting particles 32 and black particles 34. Theretroreflective particles may include retroreflecting corner cubes orhemispherically reflective coated particles, for example. Uponapplication of an appropriate voltage between electrodes 36 and 36′, theblack particles 34 may move to the viewing surface of the displaycreating a dark state. When the retroreflective particles may move tothe top surface of the display by application of a different electricfield, they create a retroreflective surface resulting in a brightstate.

In another embodiment, a display which may be made selectivelyretroreflective is described. In general, the display works bymanipulating charged particles to either block a retroreflective lightpath or create a retroreflective surface. The particles move(electrophoretically, for example) within a capsule. FIGS. 6A-6C showthe contemplated configurations.

The capsule is situated in a two or three-dimensional corner cube-typestructure, which may be created by embossing or other means. In twostates, as shown in FIGS. 6A and 6B, the particles 38 allow light 40 topass through and be reflected by the corner cube 42. In a third state,however, as shown in FIG. 6C, the particles 38 block most of theincident light 40 from being retroreflected by the corner cube 42.

In another embodiment, shown in FIG. 7, a single capsule acts as aretroreflector, much as a glass bead does. Only light that enters theincident side 44 at a vertical displacement at a distance from thecenter greater than a critical distance y will strike the totallyinternal reflecting (TIR) side 46 at an angle great enough to be totallyinternally reflected. This light strikes the TIR side near its center.Thus, on the incident side 44, the retroreflective effect occurs awayfrom the center axis. On the TIR side 46, however, most retroreflectiveaction occurs at the vertical center.

Thus, a retroreflective electronically addressable display can beconstructed in which the retroreflective and non-retroreflective statesare as illustrated in FIGS. 8A and 8B. In FIG. 8A, the particles 43 areshown toward the front face of the capsule 45. This configuration allowslight to enter and be reflected from the TIR side of the capsule. InFIG. 8B, the particles 43 are shown toward the bottom face of thecapsule 45. In this configuration, the particles block the path of thelight and thereby prevent it from being reflected from the TIR side ofthe capsule.

In short, any configuration in which particles can be rearranged in acapsule or capsular cavity in a binder, with or without an externalphysical retroreflector, to switch from a retroreflected to anon-retroreflected state is contemplated.

In another embodiment of the invention, a multi-color, encapsulatedelectrophoretic display is contemplated. In this embodiment, thedisplay, which may comprise a capsule, is filled with at least onesuspending fluid and at least two, and preferably at least three,species of particles. These particles are of different colors andpossess substantially non-overlapping electrophoretic mobilities. Asused herein, the phrase “substantially non-overlapping electrophoreticmobilities” means that less than 25%, and preferably less than 5%, ofthe particles of different colors have the same or similarelectrophoretic mobilities. As an example, in a system having twospecies of particles, less than 25% of particles of one species wouldhave the same or similar electrophoretic mobilities as the particles inthe other species. Finally, in an alternative embodiment, one of thecolors may be represented by a dye dispersed in the suspending fluid.

As an example of a multi-color, encapsulated electrophoretic display,there may be magenta particles with an average zeta potential of 100 mV,cyan particles with an average zeta potential of 60 mV, and yellowparticles with an average zeta potential of 20 mV. To address thisdisplay to the magenta state, all the particles are pulled to the backof the cell by applying an electric field in one direction.Subsequently, the field is reversed for just long enough for the magentaparticles to move to the top face of the display cell. The cyan andyellow particles will also move in this reversed field, but they willnot move as quickly as the magenta particles, and thus will be obscuredby the magenta particles.

To address the display to the cyan state, all the particles are pulledto the back of the cell by applying an electric field in one direction.Then the field is reversed for just long enough for the magenta and cyanparticles to move to the top face of the display cell. The field is thenreversed again and the magenta particles, moving faster than the cyanparticles, leave the cyan particles exposed at the top of the display.

Finally, to achieve a yellow display, all the particles are pulled tothe front of the display. The field is then reversed and the yellowparticles, lagging behind the magenta and cyan particles are exposed atthe front of the display.

A display using field effect luminescence is also an embodiment of theinvention. An example of a field effect luminescent embodiment of theinvention requires about 300-400 Hz AC voltage. This high frequency doesnot, however, produce any net displacement of the luminescent particles.The luminescent particles are generally conductive. Encapsulation in apolymer or other dielectric material is, therefore, useful for reducingconductivity.

FIGS. 9 and 10 show a display cell 48 of this embodiment in its whiteand darkened states, respectively. The luminescent particles 50 may be,for example, zinc sulfide particles, which emit light when excited by anAC electric field. The AC field can be superimposed on top of, or after,a DC field used for addressing the particles or dye. A second species ofparticle 52 in the fluid blocks the light being emitted from theparticles when the display is addressed to its darkened state.

Upon application of a DC field by the two electrodes 53, the luminescentparticles 50 migrate to the viewing face of the display 48 and areexcited to emit light, resulting in a bright state. Upon application ofan electric field of the opposite polarity, the luminescent particles 50migrate to the back face of the display 48, and the light-blockingparticles 52 block the light being emitted from the luminescentparticles 50 from the viewing face of the display, resulting in a darkstate. The luminescent particles may be photoluminescent orelectroluminescent. Photoluminescent particles may be stimulated bycontinuous ultraviolet, or other, radiation from the front of thedisplay, or the illumination source may be behind the display. In thelatter case, the dye or second species of particle allows ultravioletradiation to pass through the display.

In a preferred embodiment of the invention, the electrophoretic displaycomprises a capsule in a binder, the capsule containing a plurality ofparticles which are themselves encapsulated pigments, dyes, dispersionsor dye solutions. In this embodiment, a pigment, for example, isencapsulated to form particles ranging from tens of nanometers to a fewmicrometers, which are then dispersed and encapsulated. Examples includescattering pigments, absorbing pigments, or luminescent particles. Theseparticles are then used as the electrophoretic particles. Furthermore,in this embodiment of the invention, it is possible to encapsulate a dyesolution and use it as the electrophoretic particle.

Furthermore, in this embodiment, it is possible to encapsulate not onlya fluid dye or a particle, but also a fluid dye plus solid particles.These particles possess their own optical or electrical properties,which complement may those of the dye.

These encapsulated particles may be useful for both encapsulatedelectrophoretic displays and non-encapsulated electrophoretic displays.The average diameter for a particle is in the range of about 10 nm toabout 5 .μm in diameter. These capsules need to be small enough to bemobile within the larger capsule, which typically has a size rangingfrom about 5 .μm to about 400 .μm in diameter.

In another embodiment of the invention, an encapsulated electro-osmoticdisplay is described. In this embodiment, a porous or gel-like internalphase of a capsule is swelled (i.e., filled) and drained by theelectro-osmotically induced motion of a refractive index matching fluid(i.e., the difference between the refractive index of the fluid and therefractive index of the internal phase is preferably within 0.5). Whenthe pores of the material are filled with the fluid, the capsule acts asa homogeneous optical material, thus largely transmitting or refractinglight according to the bulk properties of the medium. When the pores arevacated by the mobile fluid, however, a larger quantity of optical indexmismatches are present and light scattering is greatly increased.

The porous internal phase of the capsule may include a cellulosicmaterial, such as an alkylcellulose. Examples of alkylcellulosesinclude, but are not limited to, methylcellulose,methylhydroxyethylcellulose, hydroxyethylcellulose,hydroxypropylmethylcellulose, carboxymethylcellulose, and sodiumcarboxymethyl-cellulose.

In other embodiments of the invention, it is preferred that the capsulesof the electrophoretic display have a non-spherical shape. There aresome optical losses associated with encapsulated electrophoreticdisplays compared to unencapsulated displays due to absorption orscattering by the capsule materials, and absorption or scattering of thebinder. Many of these losses result from spherical cavities. It is,therefore, advantageous to provide a non-spherical microcapsule,specifically a closely packed array of non-spherical cavities. Asillustrated in FIG. 14, it is desirable that the top of the microcapsule100 have a flat surface 102 that is co-planar with the viewing electrode104 and vertical, or nearly vertical, walls 106. The capsule may be aslightly flattened sphere (FIG. 15A), a heavily flattened sphere (FIG.15B), essentially cylindrical in shape, or a multi-faceted polyhedron(FIG. 15C), for example.

A display with non-spherical capsules may comprise a binder havingoil-containing cavities that are non-spherical in shape. Theseoil-containing cavities may be elastomeric capsules. In a preferredembodiment, the aspect ratio (i.e., ratio of width to height) of thesecavities is preferably greater than about 1.2. The aspect ratio is morepreferably greater than about 1.5, and, in a particularly preferredembodiment, the aspect ratio is greater than about 1.75. In a preferredembodiment, a display having non-spherical capsules has a volumefraction (i.e., fraction of total volume) of binder between about 0 toabout 0.9. More preferably, the volume fraction is between about 0.05and about 0.2.

Displays of this type have both a rear surface and a viewed surface. Ina preferred embodiment, the viewed surface is substantially planar. Asused herein, the phrase “substantially planar” means a curvature (i.e.,inverse of a radius of curvature) of less than about 2.0 m. In aparticularly preferred embodiment, both the rear and viewed surfaces aresubstantially planar. Furthermore, embodiments of such displays willpreferably have an optically active fraction (i.e., percentage of totalsurface area that can change optical properties) of greater than about80%, and more preferably greater than about 90%.

Non-spherical microcapsules can be formed during the encapsulationphase, by, for example, using a non-uniform shear field or a compressivepressure. Such non-spherical capsules can also be formed during theprocessing of the display when the binder is drying or curing. In such asystem, as the binder shrinks, it pulls capsules close to one anotherand pulls the capsules down toward the substrate on which they have beencoated. For example, an aqueous evaporative binder, such as a waterborneacrylic, urethane, or poly(vinyl alcohol), for example, tends to exhibitsuch shrinking properties. Any other evaporative binder, emulsion, orsolution would also be suitable. The solvent need not be water, but canbe an organic liquid or a combination of liquids.

Such non-spherical capsules can be formed by applying a force to thefilm as it is drying or curing to deform permanently the capsules. Sucha force can be applied by a pair of rollers, by a vacuum laminationpress, by a mechanical press, or by any other suitable means. Suchnon-spherical capsules can also be formed by stretching the cured filmin one or both of the planar axes of the film. After completion of thecuring process, the capsule can protrude above the surface of the curedfilm resulting in a lens effect that enhances the optical properties ofthe capsule. Finally, the capsule can also be of a material whichsoftens in the binder, thus allowing a flattened capsule when thecapsules and binder are laid down and the binder is allowed to cure.

In another embodiment, a polymer-dispersed electrophoretic display isconstructed in a manner similar to a polymer-dispersed liquid crystaldisplay. As the binder dries or cures, the encapsulated phase is pulledinto non-spherical cavities.

An electrophoretic display is constructed as either an encapsulatedelectrophoretic display or a polymer-dispersed electrophoretic display(similar in construction to a polymer dispersed liquid crystal display),and the capsules or liquid droplets are formed non-spherically, byflattening, by shrinkage of the binder, or by mechanical force. In eachcase, the capsules should be capable of deforming, or they may rupture.In the case of a polymer-dispersed electrophoretic display, theencapsulated phases change shape as the polymer shrinks. In addition,the encapsulated phases may be deformed asymmetrically by stretching thesubstrate. Another technique which may be employed is to first dry thebinder in such a way that a tough top skin is formed. The rest of thebinder may then be dried slowly with no fear of the top surface breakingor becoming too uneven.

The electrodes adjacent the electrophoretic display may includeconducting polymers, such as polyaniline, for example. These materialsmay be soluble, enabling them to be coated by, for example, web coating.

A means for addressing an encapsulated electrophoretic display (or otherdisplay) structure is also described. Referring to FIG. 11, electrodes66 and 66′ are present on one side of the display. These electrodes maybe part of a head (“stylus”) which is scanned over the display. Theremay be more, less, or exactly one capsule per electrode pair.Application of a DC field to the pixel moves the particles to one sideand exposes substrate 68 beneath (e.g., a mirror, a retroflectivecoating, a diffuse reflective coating, etc.). Under the influence of anAC field, the particles 70 may be distributed throughout the space andcause a largely dark pixel appearance. The electrodes themselves may beclear or opaque.

Referring to FIGS. 12A and 12B, a similar structure is described. Theelectrodes 72 and 72′ are, however, different in size (e.g., by morethan a factor of 2). The particles are moved to mask either electrode bychanging the electric field polarity. In one case (FIG. 12A), theparticles cover a small area, and the pixel is largely reflective. Inthe other case (FIG. 12B), the particles 74′ cover a large area, and thepixel is largely absorbing. The materials may be reversed, for examplereflective particles and an absorbing backing. There may be a mask whichcovers one of the electrode locations on the material.

This method of addressing a display includes writing with anelectrostatic head either permanently mounted in a fixture or hand held.It may also be applied to an encapsulated magnetophoretic material. Itmay also be applied to a polymer stabilized liquid crystal technology,or a bistable liquid crystal material of any type, for example, nematicon a photoaligned layer. It may also be applied to a suspended particledisplay.

Referring again to both FIGS. 11 and 12, in either embodiment, the rearelectrode may also be provided as a transmissive, translucent, orotherwise transparent backing, instead of reflective. In theseembodiments, a DC field may cause dark (absorbing) particles to coverone electrode, as described above, and the pixel is largelytransmissive. These embodiments allow the display to be used to“shutter” light. For example, a display including the described capsulesmay be addressed so that all of the pixels present in the display arelargely transparent, in which case the display would act as a window orclear device. Alternatively, if a fraction of the capsules areaddressed, the display is partially transmissive. If all of the capsulesare addressed using an AC field, the display is either opaque orreflective.

II. MATERIALS FOR USE IN ELECTROPHORETIC DISPLAYS

Useful materials for constructing the above-described encapsulatedelectrophoretic displays are discussed below. Many of these materialswill be known to those skilled in the art of constructing conventionalelectrophoretic displays, or those skilled in the art ofmicroencapsulation. The combination of these materials and processes,along with the other necessary components found in an encapsulatedelectrophoretic display, comprise the invention described herein.

A. Particles

There is much flexibility in the choice of particles for use inelectrophoretic displays, as described above. For purposes of thisinvention, a particle is any component that is charged or capable ofacquiring a charge (i.e., has or is capable of acquiring electrophoreticmobility), and, in some cases, this mobility may be zero or close tozero (i.e., the particles will not move). The particles may be neatpigments, dyed (laked) pigments or pigment/polymer composites, or anyother component that is charged or capable of acquiring a charge.Typical considerations for the electrophoretic particle are its opticalproperties, electrical properties, and surface chemistry. The particlesmay be organic or inorganic compounds, and they may either absorb lightor scatter light. The particles for use in the invention may furtherinclude scattering pigments, absorbing pigments and luminescentparticles. The particles may be retroreflective, such as corner cubes,or they may be electroluminescent, such as zinc sulfide particles, whichemit light when excited by an AC field, or they may be photoluminescent.Finally, the particles may be surface treated so as to improve chargingor interaction with a charging agent, or to improve dispersibility.

A preferred particle for use in electrophoretic displays of theinvention is titania. The titania particles may be coated with a metaloxide, such as aluminum oxide or silicon oxide, for example. The titaniaparticles may have one, two, or more layers of metal-oxide coating. Forexample, a titania particle for use in electrophoretic displays of theinvention may have a coating of aluminum oxide and a coating of siliconoxide. The coatings may be added to the particle in any order.

The electrophoretic particle is usually a pigment, a polymer, a lakedpigment, or some combination of the above. A neat pigment can be anypigment, and, usually for a light colored particle, pigments such as,for example, rutile (titania), anatase (titania), barium sulfate,kaolin, or zinc oxide are useful. Some typical particles have highrefractive indices, high scattering coefficients, and low absorptioncoefficients. Other particles are absorptive, such as carbon black orcolored pigments used in paints and inks. The pigment should also beinsoluble in the suspending fluid. Yellow pigments such as diarylideyellow, hansa yellow, and benzidin yellow have also found use in similardisplays. Any other reflective material can be employed for a lightcolored particle, including non-pigment materials, such as metallicparticles.

Useful neat pigments include, but are not limited to, PbCrO₄, Cyan blueGT 55-3295 (American Cyanamid Company, Wayne, N.J.), Cibacron Black BG(Ciba Company, Inc., Newport, Del.), Cibacron Turquoise Blue G (Ciba),Cibalon Black BGL (Ciba), Orasol Black BRG (Ciba), Orasol Black RBL(Ciba), Acetamine Blac, CBS (E. I. du Pont de Nemours and Company, Inc.,Wilmington, Del.), Crocein Scarlet N Ex (E. I. du Pont de Nemours andCompany) (27290), Fiber Black VF (E. I. du Pont de Nemours and Company)(30235), Luxol Fast Black L (E. I. du Pont de Nemours and Company)(Solv. Black 17), Nirosine Base No. 424 (E. I. du Pont de Nemours andCompany) (50415 B), Oil Black BG (E. I. du Pont de Nemours and Company)(Solv. Black 16), Rotalin Black RM (E. I. du Pont de Nemours andCompany), Sevron Brilliant Red 3 B (E. I. du Pont de Nemours andCompany); Basic Black DSC (Dye Specialties, Inc.), Hectolene Black (DyeSpecialties, Inc.), Azosol Brilliant Blue B (GAF, Dyestuff and ChemicalDivision, Wayne, N.J.) (Solv. Blue 9), Azosol Brilliant Green BA (GAF)(Solv. Green 2), Azosol Fast Brilliant Red B (GAF), Azosol Fast OrangeRA Conc. (GAF) (Solv. Orange 20), Azosol Fast Yellow GRA Conc. (GAF)(13900 A), Basic Black KMPA (GAF), Benzofix Black CW-CF (GAF) (35435),Cellitazol BNFV Ex Soluble CF (GAF) (Disp. Black 9), Celliton Fast BlueAF Ex Conc (GAF) (Disp. Blue 9), Cyper Black IA (GAF) (Basic Blk. 3),Diamine Black CAP Ex Conc (GAF) (30235), Diamond Black EAN Hi Con. CF(GAF) (15710), Diamond Black PBBA Ex (GAF) (16505); Direct Deep Black EAEx CF (GAF) (30235), Hansa Yellow G (GAF) (11680); Indanthrene Black BBKPowd. (GAF) (59850), Indocarbon CLGS Conc. CF (GAF) (53295), KatigenDeep Black NND Hi Conc. CF (GAF) (15711), Rapidogen Black 3 G (GAF)(Azoic Blk. 4); Sulphone Cyanine Black BA-CF (GAF) (26370), ZambeziBlack VD Ex Conc. (GAF) (30015); Rubanox Red CP-1495 (TheSherwin-Williams Company, Cleveland, Ohio) (15630); Raven 11 (ColumbianCarbon Company, Atlanta, Ga.), (carbon black aggregates with a particlesize of about 25 .μm), Statex B-12 (Columbian Carbon Co.) (a furnaceblack of 33 .μm average particle size), and chrome green.

Particles may also include laked, or dyed, pigments. Laked pigments areparticles that have a dye precipitated on them or which are stained.Lakes are metal salts of readily soluble anionic dyes. These are dyes ofazo, triphenylmethane or anthraquinone structure containing one or moresulphonic or carboxylic acid groupings. They are usually precipitated bya calcium, barium or aluminium salt onto a substrate. Typical examplesare peacock blue lake (CI Pigment Blue 24) and Persian orange (lake ofCI Acid Orange 7), Black M Toner (GAF) (a mixture of carbon black andblack dye precipitated on a lake).

A dark particle of the dyed type may be constructed from any lightabsorbing material, such as carbon black, or inorganic black materials.The dark material may also be selectively absorbing. For example, a darkgreen pigment may be used. Black particles may also be formed bystaining latices with metal oxides, such latex copolymers consisting ofany of butadiene, styrene, isoprene, methacrylic acid, methylmethacrylate, acrylonitrile, vinyl chloride, acrylic acid, sodiumstyrene sulfonate, vinyl acetate, chlorostyrene,dimethylaminopropylmethacrylamide, isocyanoethyl methacrylate andN-(isobutoxymethacrylamide), and optionally including conjugated dienecompounds such as diacrylate, triacrylate, dimethylacrylate andtrimethacrylate. Black particles may also be formed by a dispersionpolymerization technique.

In the systems containing pigments and polymers, the pigments andpolymers may form multiple domains within the electrophoretic particle,or be aggregates of smaller pigment/polymer combined particles.Alternatively, a central pigment core may be surrounded by a polymershell. The pigment, polymer, or both can contain a dye. The opticalpurpose of the particle may be to scatter light, absorb light, or both.Useful sizes may range from 1 nm up to about 100 .μm, as long as theparticles are smaller than the bounding capsule. The density of theelectrophoretic particle may be substantially matched to that of thesuspending (i.e., electrophoretic) fluid. As defined herein, asuspending fluid has a density that is “substantially matched” to thedensity of the particle if the difference in their respective densitiesis between about zero and about two g/ml. This difference is preferablybetween about zero and about 0.5 g/ml.

Useful polymers for the particles include, but are not limited to:polystyrene, polyethylene, polypropylene, phenolic resins, E. I. du Pontde Nemours and Company Elvax resins (ethylene-vinyl acetate copolymers),polyesters, polyacrylates, polymethacrylates, ethylene acrylic acid ormethacrylic acid copolymers (Nucrel Resins—E. I. du Pont de Nemours andCompany, Primacor Resins—Dow Chemical), acrylic copolymers andterpolymers (Elvacite Resins, E. I. du Pont de Nemours and Company) andPMMA. Useful materials for homopolymer/pigment phase separation in highshear melt include, but are not limited to, polyethylene, polypropylene,polymethylmethacrylate, polyisobutylmethacrylate, polystyrene,polybutadiene, polyisoprene, polyisobutylene, polylauryl methacrylate,polystearyl methacrylate, polyisobornyl methacrylate, poly-t-butylmethacrylate, polyethyl methacrylate, polymethyl acrylate, polyethylacrylate, polyacrylonitrile, and copolymers of two or more of thesematerials. Some useful pigment/polymer complexes that are commerciallyavailable include, but are not limited to, Process Magenta PM 1776(Magruder Color Company, Inc., Elizabeth, N.J.), Methyl Violet PMAVM6223 (Magruder Color Company, Inc., Elizabeth, N.J.), and Naphthol FGRRF6257 (Magruder Color Company, Inc., Elizabeth, N.J.).

The pigment-polymer composite may be formed by a physical process,(e.g., attrition or ball milling), a chemical process (e.g.,microencapsulation or dispersion polymerization), or any other processknown in the art of particle production. From the following non-limitingexamples, it may be seen that the processes and materials for both thefabrication of particles and the charging thereof are generally derivedfrom the art of liquid toner, or liquid immersion development. Thus anyof the known processes from liquid development are particularly, but notexclusively, relevant.

New and useful electrophoretic particles may still be discovered, but anumber of particles already known to those skilled in the art ofelectrophoretic displays and liquid toners can also prove useful. Ingeneral, the polymer requirements for liquid toners and encapsulatedelectrophoretic inks are similar, in that the pigment or dye must beeasily incorporated therein, either by a physical, chemical, orphysicochemical process, may aid in the colloidal stability, and maycontain charging sites or may be able to incorporate materials whichcontain charging sites. One general requirement from the liquid tonerindustry that is not shared by encapsulated electrophoretic inks is thatthe toner must be capable of “fixing” the image, i.e., heat fusingtogether to create a uniform film after the deposition of the tonerparticles.

Typical manufacturing techniques for particles are drawn from the liquidtoner and other arts and include ball milling, attrition, jet milling,etc. The process will be illustrated for the case of a pigmentedpolymeric particle. In such a case the pigment is compounded in thepolymer, usually in some kind of high shear mechanism such as a screwextruder. The composite material is then (wet or dry) ground to astarting size of around 10 .μm. It is then dispersed in a carrierliquid, for example ISOPAR® (Exxon, Houston, Tex.), optionally with somecharge control agent(s), and milled under high shear for several hoursdown to a final particle size and/or size distribution.

Another manufacturing technique for particles drawn from the liquidtoner field is to add the polymer, pigment, and suspending fluid to amedia mill. The mill is started and simultaneously heated to temperatureat which the polymer swells substantially with the solvent. Thistemperature is typically near 100° C. In this state, the pigment iseasily encapsulated into the swollen polymer. After a suitable time,typically a few hours, the mill is gradually cooled back to ambienttemperature while stirring. The milling may be continued for some timeto achieve a small enough particle size, typically a few microns indiameter. The charging agents may be added at this time. Optionally,more suspending fluid may be added.

Chemical processes such as dispersion polymerization, mini- ormicro-emulsion polymerization, suspension polymerization precipitation,phase separation, solvent evaporation, in situ polymerization, seededemulsion polymerization, or any process which falls under the generalcategory of microencapsulation may be used. A typical process of thistype is a phase separation process wherein a dissolved polymericmaterial is precipitated out of solution onto a dispersed pigmentsurface through solvent dilution, evaporation, or a thermal change.Other processes include chemical means for staining polymeric latices,for example with metal oxides or dyes.

B. Suspending Fluid

The suspending fluid containing the particles can be chosen based onproperties such as density, refractive index, and solubility. Apreferred suspending fluid has a low dielectric constant (about 2), highvolume resistivity (about 10̂15 ohm-cm), low viscosity (less than 5 cst),low toxicity and environmental impact, low water solubility (less than10 ppm), high specific gravity (greater than 1.5), a high-boiling point(greater than 90° C.), and a low refractive index (less than 1.2).

The choice of suspending fluid may be based on concerns of chemicalinertness, density matching to the electrophoretic particle, or chemicalcompatibility with both the electrophoretic particle and boundingcapsule. The viscosity of the fluid should be low when you want theparticles to move. The refractive index of the suspending fluid may alsobe substantially matched to that of the particles. As used herein, therefractive index of a suspending fluid “is substantially matched” tothat of a particle if the difference between their respective refractiveindices is between about zero and about 0.3, and is preferably betweenabout 0.05 and about 0.2.

Additionally, the fluid may be chosen to be a poor solvent for somepolymers, which is advantageous for use in the fabrication ofmicroparticles because it increases the range of polymeric materialsuseful in fabricating particles of polymers and pigments. Organicsolvents, such as halogenated organic solvents, saturated linear orbranched hydrocarbons, silicone oils, and low molecular weighthalogen-containing polymers are some useful suspending fluids. Thesuspending fluid may comprise a single fluid. The fluid will, however,often be a blend of more than one fluid in order to tune its chemicaland physical properties. Furthermore, the fluid may contain surfacemodifiers to modify the surface energy or charge of the electrophoreticparticle or bounding capsule. Reactants or solvents for themicroencapsulation process (oil soluble monomers, for example) can alsobe contained in the suspending fluid. Charge control agents can also beadded to the suspending fluid.

Useful organic solvents include, but are not limited to, epoxides, suchas, for example, decane epoxide and dodecane epoxide; vinyl ethers, suchas, for example, cyclohexyl vinyl ether and Decave® (InternationalFlavors & Fragrances, Inc., New York, N.Y.); and aromatic hydrocarbons,such as, for example, toluene and naphthalene. Useful halogenatedorganic solvents include, but are not limited to,tetrafluorodibromoethylene, tetrachloroethylene,trifluorochloroethylene, 1,2,4-trichlorobenzene, carbon tetrachloride.These materials have high densities. Useful hydrocarbons include, butare not limited to, dodecane, tetradecane, the aliphatic hydrocarbons inthe Isopar series (Exxon, Houston, Tex.), Norpar (series of normalparaffinic liquids), Shell-Sol® (Shell, Houston, Tex.), and Sol-Trol®(Shell), naphtha, and other petroleum solvents. These materials usuallyhave low densities. Useful examples of silicone oils include, but arenot limited to, octamethyl cyclosiloxane and higher molecular weightcyclic siloxanes, poly (methyl phenyl siloxane), hexamethyldisiloxane,and polydimethylsiloxane. These materials usually have low densities.Useful low molecular weight halogen-containing polymers include, but arenot limited to, poly(chlorotrifluoroethylene) polymer (Halogenatedhydrocarbon Inc., River Edge, N.J.), Galden® (a perfluorinated etherfrom Ausimont, Morristown, N.J.), or Krytox from E. I. du Pont deNemours and Company (Wilmington, Del.). In a preferred embodiment, thesuspending fluid is a poly(chlorotrifluoroethylene) polymer. In aparticularly preferred embodiment, this polymer has a degree ofpolymerization from about 2 to about 10. Many of the above materials areavailable in a range of viscosities, densities, and boiling points.

The fluid must be capable of being formed into small droplets prior to acapsule being formed. Processes for forming small droplets includeflow-through jets, membranes, nozzles, or orifices, as well asshear-based emulsifying schemes. The formation of small drops may beassisted by electrical or sonic fields. Surfactants and polymers can beused to aid in the stabilization and emulsification of the droplets inthe case of an emulsion type encapsulation. A preferred surfactant foruse in displays of the invention is sodium dodecylsulfate.

It can be advantageous in some displays for the suspending fluid tocontain an optically absorbing dye. This dye must be soluble in thefluid, but will generally be insoluble in the other components of thecapsule. There is much flexibility in the choice of dye material. Thedye can be a pure compound, or blends of dyes to achieve a particularcolor, including black. The dyes can be fluorescent, which would producea display in which the fluorescence properties depend on the position ofthe particles. The dyes can be photoactive, changing to another color orbecoming colorless upon irradiation with either visible or ultravioletlight, providing another means for obtaining an optical response. Dyescould also be polymerizable, forming a solid absorbing polymer insidethe bounding shell.

There are many dyes that can be chosen for use in encapsulatedelectrophoretic display. Properties important here include lightfastness, solubility in the suspending liquid, color, and cost. Theseare generally from the class of azo, anthraquinone, and triphenylmethanetype dyes and may be chemically modified so as to increase thesolubility in the oil phase and reduce the adsorption by the particlesurface.

A number of dyes already known to those skilled in the art ofelectrophoretic displays will prove useful. Useful azo dyes include, butare not limited to: the Oil Red dyes, and the Sudan Red and Sudan Blackseries of dyes. Useful anthraquinone dyes include, but are not limitedto: the Oil Blue dyes, and the Macrolex Blue series of dyes. Usefultriphenylmethane dyes include, but are not limited to, Michler's hydrol,Malachite Green, Crystal Violet, and Auramine O.

C. Charge Control Agents and Particle Stabilizers

Charge control agents are used to provide good electrophoretic mobilityto the electrophoretic particles. Stabilizers are used to preventagglomeration of the electrophoretic particles, as well as prevent theelectrophoretic particles from irreversibly depositing onto the capsulewall. Either component can be constructed from materials across a widerange of molecular weights (low molecular weight, oligomeric, orpolymeric), and may be pure or a mixture. In particular, suitable chargecontrol agents are generally adapted from the liquid toner art. Thecharge control agent used to modify and/or stabilize the particlesurface charge is applied as generally known in the arts of liquidtoners, electrophoretic displays, non-aqueous paint dispersions, andengine-oil additives. In all of these arts, charging species may beadded to non-aqueous media in order to increase electrophoretic mobilityor increase electrostatic stabilization. The materials can improvesteric stabilization as well. Different theories of charging arepostulated, including selective ion adsorption, proton transfer, andcontact electrification.

An optional charge control agent or charge director may be used. Theseconstituents typically consist of low molecular weight surfactants,polymeric agents, or blends of one or more components and serve tostabilize or otherwise modify the sign and/or magnitude of the charge onthe electrophoretic particles. The charging properties of the pigmentitself may be accounted for by taking into account the acidic or basicsurface properties of the pigment, or the charging sites may take placeon the carrier resin surface (if present), or a combination of the two.Additional pigment properties which may be relevant are the particlesize distribution, the chemical composition, and the lightfastness. Thecharge control agent used to modify and/or stabilize the particlesurface charge is applied as generally known in the arts of liquidtoners, electrophoretic displays, non-aqueous paint dispersions, andengine-oil additives. In all of these arts, charging species may beadded to non-aqueous media in order to increase electrophoretic mobilityor increase electrostatic stabilization. The materials can improvesteric stabilization as well. Different theories of charging arepostulated, including selective ion adsorption, proton transfer, andcontact electrification.

Charge adjuvants may also be added. These materials increase theeffectiveness of the charge control agents or charge directors. Thecharge adjuvant may be a polyhydroxy compound or an aminoalcoholcompound, which are preferably soluble in the suspending fluid in anamount of at least 2% by weight. Examples of polyhydroxy compounds whichcontain at least two hydroxyl groups include, but are not limited to,ethylene glycol, 2,4,7,9-tetramethyl-decyne-4,7-diol, poly (propyleneglycol), pentaethylene glycol, tripropylene glycol, triethylene glycol,glycerol, pentaerythritol, glycerol-tris-12-hydroxystearate, propyleneglycerol monohydroxystearate, and ethylene glycol monohydroxystearate.Examples of aminoalcohol compounds which contain at least one alcoholfunction and one amine function in the same molecule include, but arenot limited to, triisopropanolamine, triethanolamine, ethanolamine,3-amino-1-propanol, o-aminophenol, 5-amino-1-pentanol, andtetrakis(2-hydroxyethyl)ethylene-diamine. The charge adjuvant ispreferably present in the suspending fluid in an amount of about 1 toabout 100 mg/g of the particle mass, and more preferably about 50 toabout 200 mg/g.

The surface of the particle may also be chemically modified to aiddispersion, to improve surface charge, and to improve the stability ofthe dispersion, for example. Surface modifiers include organicsiloxanes, organohalogen silanes and other functional silane couplingagents (Dow Corning® Z-6070, Z-6124, and 3 additive, Midland, Miss.);organic titanates and zirconates (Tyzor® TOT, TBT, and TE Series, E. I.du Pont de Nemours and Company, Wilmington, Del.); hydrophobing agents,such as long chain (C12 to C50) alkyl and alkyl benzene sulphonic acids,fatty amines or diamines and their salts or quaternary derivatives; andamphipathic polymers which can be covalently bonded to the particlesurface.

In general, it is believed that charging results as an acid-basereaction between some moiety present in the continuous phase and theparticle surface. Thus useful materials are those which are capable ofparticipating in such a reaction, or any other charging reaction asknown in the art.

Different non-limiting classes of charge control agents which are usefulinclude organic sulfates or sulfonates, metal soaps, block or combcopolymers, organic amides, organic zwitterions, and organic phosphatesand phosphonates. Useful organic sulfates and sulfonates include, butare not limited to, sodium bis(2-ethyl-hexyl) sodium sulfosuccinate,calcium dodecyl benzene sulfonate, calcium petroleum sulfonate, neutralor basic barium dinonylnaphthalene sulfonate, neutral or basic calciumdinonylnaphthalene sulfonate, dodecylbenzenesulfonic acid sodium salt,and ammonium lauryl sulphate. Useful metal soaps include, but are notlimited to, basic or neutral barium petronate, calcium petronate, Co—,Ca—, Cu—, Mn—, Ni—, Zn—, and Fe— salts of naphthenic acid, Ba—, Al—,Zn—, Cu—, Pb—, and Fe— salts of stearic acid, divalent and trivalentmetal carboxylates, such as aluminum tristearate, aluminum octanoate,lithium heptanoate, iron stearate, iron distearate, barium stearate,chromium stearate, magnesium octanoate, calcium stearate, ironnaphthenate, and zinc naphthenate, Mn— and Zn— heptanoate, and Ba—, Al—,Co—, Mn—, and Zn— octanoate. Useful block or comb copolymers include,but are not limited to, AB diblock copolymers of (A) polymers of2-(N,N)-dimethylaminoethyl methacrylate quaternized withmethyl-p-toluenesulfonate and (B) poly-2-ethylhexyl methacrylate, andcomb graft copolymers with oil soluble tails of poly (12-hydroxystearicacid) and having a molecular weight of about 1800, pendant on anoil-soluble anchor group of poly (methyl methacrylate-methacrylic acid).Useful organic amides include, but are not limited to, polyisobutylenesuccinimides such as OLOA 1200, and N-vinyl pyrrolidone polymers. Usefulorganic zwitterions include, but are not limited to, lecithin. Usefulorganic phosphates and phosphonates include, but are not limited to, thesodium salts of phosphated mono- and di-glycerides with saturated andunsaturated acid substituents.

Particle dispersion stabilizers may be added to prevent particleflocculation or attachment to the capsule walls. For the typical highresistivity liquids used as suspending fluids in electrophoreticdisplays, nonaqueous surfactants may be used. These include, but are notlimited to, glycol ethers, acetylenic glycols, alkanolamides, sorbitolderivatives, alkyl amines, quaternary amines, imidazolines, dialkyloxides, and sulfosuccinates.

D. Encapsulation

There is a long and rich history to encapsulation, with numerousprocesses and polymers having proven useful in creating capsules.Encapsulation of the internal phase may be accomplished in a number ofdifferent ways. Numerous suitable procedures for microencapsulation aredetailed in both Microencapsulation, Processes and Applications, (I. E.Vandegaer, ed.), Plenum Press, New York, N.Y. (1974) and Gutcho,Microcapsules and Microencapsulation Techniques, Nuyes Data Corp., ParkRidge, N.J. (1976), both of which are hereby incorporated by referenceherein. The processes fall into several general categories, all of whichcan be applied to the present invention: interfacial polymerization, insitu polymerization, physical processes, such as coextrusion and otherphase separation processes, in-liquid curing, and simple/complexcoacervation.

Numerous materials and processes should prove useful in formulatingdisplays of the present invention. Useful materials for simplecoacervation processes include, but are not limited to, gelatin,polyvinyl alcohol, polyvinyl acetate, and cellulosic derivatives, suchas, for example, carboxymethylcellulose. Useful materials for complexcoacervation processes include, but are not limited to, gelatin, acacia,carageenan, carboxymethylcellulose, hydrolized styrene anhydridecopolymers, agar, alginate, casein, albumin, methyl vinyl etherco-maleic anhydride, and cellulose phthalate. Useful materials for phaseseparation processes include, but are not limited to, polystyrene, PMMA,polyethyl methacrylate, polybutyl methacrylate, ethyl cellulose,polyvinyl pyridine, and poly acrylonitrile. Useful materials for in situpolymerization processes include, but are not limited to,polyhydroxyamides, with aldehydes, melamine, or urea and formaldehyde;water-soluble oligomers of the condensate of melamine, or urea andformaldehyde; and vinyl monomers, such as, for example, styrene, MMA andacrylonitrile. Finally, useful materials for interfacial polymerizationprocesses include, but are not limited to, diacyl chlorides, such as,for example, sebacoyl, adipoyl, and di- or poly-amines or alcohols, andisocyanates. Useful emulsion polymerization materials may include, butare not limited to, styrene, vinyl acetate, acrylic acid, butylacrylate, t-butyl acrylate, methyl methacrylate, and butyl methacrylate.

Capsules produced may be dispersed into a curable carrier, resulting inan ink which may be printed or coated on large and arbitrarily shaped orcurved surfaces using conventional printing and coating techniques.

In the context of the present invention, one skilled in the art willselect an encapsulation procedure and wall material based on the desiredcapsule properties. These properties include the distribution of capsuleradii; electrical, mechanical, diffusion, and optical properties of thecapsule wall; and chemical compatibility with the internal phase of thecapsule.

The capsule wall generally has a high electrical resistivity. Althoughit is possible to use walls with relatively low resistivities, this maylimit performance in requiring relatively higher addressing voltages. Afull discussion of the relevant electrical properties of the capsulewall is set forth in U.S. Pat. No. 4,605,284, the entire disclosure ofwhich is hereby incorporated by reference herein. The capsule wallshould also be mechanically strong (although if the finished capsulepowder is to be dispersed in a curable polymeric binder for coating,mechanical strength is not as critical). The capsule wall shouldgenerally not be porous. If, however, it is desired to use anencapsulation procedure that produces porous capsules, these can beovercoated in a post-processing step (i.e., a second encapsulation).Moreover, if the capsules are to be dispersed in a curable binder, thebinder will serve to close the pores. The capsule walls should beoptically clear. The wall material may, however, be chosen to match therefractive index of the internal phase of the capsule (i.e., thesuspending fluid) or a binder in which the capsules are to be dispersed.For some applications (e.g., interposition between two fixedelectrodes), monodispersed capsule radii are desirable.

An encapsulation technique that is highly suited to the presentinvention is set forth in U.S. Pat. No. 4,087,376, the entire disclosureof which is hereby incorporated by reference herein. The procedureinvolves a polymerization between urea and formaldehyde in an aqueousphase of an oil/water emulsion in the presence of a negatively charged,carboxyl-substituted, linear hydrocarbon polyelectrolyte material. Theresulting capsule wall is a urea/formaldehyde copolymer, whichdiscretely encloses the internal phase. The capsule is clear,mechanically strong, and has good resistivity properties.

The related technique of in situ polymerization utilizes an oil/wateremulsion, which is formed by dispersing the electrophoretic composition(i.e., the dielectric liquid containing a suspension of the pigmentparticles) in an aqueous environment. The monomers polymerize to form apolymer with higher affinity for the internal phase than for the aqueousphase, thus condensing around the emulsified oily droplets. In oneespecially useful in situ polymerization processes, urea andformaldehyde condense in the presence of poly(acrylic acid) (See, e.g.,U.S. Pat. No. 4,001,140). In other useful process, described in U.S.Pat. No. 4,273,672, any of a variety of cross-linking agents borne inaqueous solution is deposited around microscopic oil droplets. Suchcross-linking agents include aldehydes, especially formaldehyde,glyoxal, or glutaraldehyde; alum; zirconium salts; and poly isocyanates.The entire disclosures of the U.S. Pat. Nos. 4,001,140 and 4,273,672patents are hereby incorporated by reference herein.

The coacervation approach also utilizes an oil/water emulsion. One ormore colloids are coacervated (i.e., agglomerated) out of the aqueousphase and deposited as shells around the oily droplets through controlof temperature, pH and/or relative concentrations, thereby creating themicrocapsule. Materials suitable for coacervation include gelatins andgum arabic. See, e.g., U.S. Pat. No. 2,800,457, the entire disclosure ofwhich is hereby incorporated by reference herein.

The interfacial polymerization approach relies on the presence of anoil-soluble monomer in the electrophoretic composition, which once againis present as an emulsion in an aqueous phase. The monomers in theminute hydrophobic droplets react with a monomer introduced into theaqueous phase, polymerizing at the interface between the droplets andthe surrounding aqueous medium and forming shells around the droplets.Although the resulting walls are relatively thin and may be permeable,this process does not require the elevated temperatures characteristicof some other processes, and therefore affords greater flexibility interms of choosing the dielectric liquid.

FIG. 13A illustrates an exemplary apparatus and environment forperforming emulsion-based encapsulation. An oil/water emulsion, isprepared in a vessel 76 equipped with a device 78 for monitoring and adevice 80 for controlling the temperature. A pH monitor 82 may also beincluded. An impeller 84 maintains agitation throughout theencapsulation process, and in combination with emulsifiers, can be usedto control the size of the emulsion droplets 86 that will lead to thefinished capsules. The aqueous continuous phase 88 may contain, forexample, a prepolymer and various system modifiers.

FIG. 13B illustrates an oil drop 90 comprising a substantiallytransparent suspending fluid 92, in which is dispersed whitemicroparticles 94 and black particles 96. Preferably, particles 94 and96 have densities substantially matched to the density of suspendingfluid 92. The liquid phase may also contain some threshold/bistabilitymodifiers, charge control agents, and/or hydrophobic monomers to effectan interfacial polymerization.

FIG. 13C illustrates a similar oil drop 98 comprising a darkly dyedsuspending fluid 100 containing a dispersion of white particles 94 andappropriate charge control agents.

Coating aids can be used to improve the uniformity and quality of thecoated or printed electrophoretic ink material. Wetting agents aretypically added to adjust the interfacial tension at thecoating/substrate interface and to adjust the liquid/air surfacetension. Wetting agents include, but are not limited to, anionic andcationic surfactants, and nonionic species, such as silicone orfluoropolymer based materials. Dispersing agents may be used to modifythe interfacial tension between the capsules and binder, providingcontrol over flocculation and particle settling.

Surface tension modifiers can be added to adjust the air/ink interfacialtension. Polysiloxanes are typically used in such an application toimprove surface levelling while minimizing other defects within thecoating. Surface tension modifiers include, but are not limited to,fluorinated surfactants, such as, for example, the Zonyl® series from E.I. du Pont de Nemours and Company (Wilmington, Del.), the Fluorod®series from 3M (St. Paul, Minn.); and the fluoroakyl series fromAutochem (Glen Rock, N.J.); siloxanes, such as, for example, Silwet®from Union Carbide (Danbury, Conn.); and polyethoxy and polypropoxyalcohols. Antifoams, such as silicone and silicone-free polymericmaterials, may be added to enhance the movement of air from within theink to the surface and to facilitate the rupture of bubbles at thecoating surface. Other useful antifoams include, but are not limited to,glyceryl esters, polyhydric alcohols, compounded antifoams, such as oilsolutions of alkyl benzenes, natural fats, fatty acids, and metallicsoaps, and silicone antifoaming agents made from the combination ofdimethyl siloxane polymers and silica. Stabilizers such as uv-absorbersand antioxidants may also be added to improve the lifetime of the ink.

Other additives to control properties like coating viscosity and foamingcan also be used in the coating fluid. Stabilizers (UV-absorbers,antioxidants) and other additives which could prove useful in practicalmaterials.

E. Binder Material

The binder is used as a non-conducting, adhesive medium supporting andprotecting the capsules, as well as binding the electrode materials tothe capsule dispersion. Binders are available in many forms and chemicaltypes. Among these are water-soluble polymers, water-borne polymers,oil-soluble polymers, thermoset and thermoplastic polymers, andradiation-cured polymers.

Among the water-soluble polymers are the various polysaccharides, thepolyvinyl alcohols, N-methyl-pyrrolidone, N-vinyl-pyrrolidone, thevarious Carbowax® species (Union Carbide, Danbury, Conn.), andpoly-2-hydroxyethylacrylate.

The water-dispersed or water-borne systems are generally latexcompositions, typified by the Neorez® and Neocryl® resins (ZenecaResins, Wilmington, Mass.), Acrysol® (Rohm and Haas, Philadelphia, Pa.),Bayhydrol® (Bayer, Pittsburgh, Pa.), and the Cytec Industries (WestPaterson, N.J.) HP line. These are generally latices of polyurethanes,occasionally compounded with one or more of the acrylics, polyesters,polycarbonates or silicones, each lending the final cured resin in aspecific set of properties defined by glass transition temperature,degree of “tack,” softness, clarity, flexibility, water permeability andsolvent resistance, elongation modulus and tensile strength,thermoplastic flow, and solids level. Some water-borne systems can bemixed with reactive monomers and catalyzed to form more complex resins.Some can be further cross-linked by the use of a crosslinking reagent,such as an aziridine, for example, which reacts with carboxyl groups.

A typical application of a water-borne resin and aqueous capsulesfollows. A volume of particles is centrifuged at low speed to separateexcess water. After a given centrifugation process, for example 10minutes at 60×G, the capsules are found at the bottom of the centrifugetube, while the water portion is at the top. The water portion iscarefully removed (by decanting or pipetting). The mass of the remainingcapsules is measured, and a mass of resin is added such that the mass ofresin is between one eighth and one tenth of the weight of the capsules.This mixture is gently mixed on an oscillating mixer for approximatelyone half hour. After about one half hour, the mixture is ready to becoated onto the appropriate substrate.

The thermoset systems are exemplified by the family of epoxies. Thesebinary systems can vary greatly in viscosity, and the reactivity of thepair determines the “pot life” of the mixture. If the pot life is longenough to allow a coating operation, capsules may be coated in anordered arrangement in a coating process prior to the resin curing andhardening.

Thermoplastic polymers, which are often polyesters, are molten at hightemperatures. A typical application of this type of product is hot-meltglue. A dispersion of heat-resistant capsules could be coated in such amedium. The solidification process begins during cooling, and the finalhardness, clarity and flexibility are affected by the branching andmolecular weight of the polymer.

Oil or solvent-soluble polymers are often similar in composition to thewater-borne system, with the obvious exception of the water itself. Thelatitude in formulation for solvent systems is enormous, limited only bysolvent choices and polymer solubility. Of considerable concern insolvent-based systems is the viability of the capsule itself—theintegrity of the capsule wall cannot be compromised in any way by thesolvent.

Radiation cure resins are generally found among the solvent-basedsystems. Capsules may be dispersed in such a medium and coated, and theresin may then be cured by a timed exposure to a threshold level ofultraviolet radiation, either long or short wavelength. As in all casesof curing polymer resins, final properties are determined by thebranching and molecular weights of the monomers, oligomers andcrosslinkers.

A number of “water-reducible” monomers and oligomers are, however,marketed. In the strictest sense, they are not water soluble, but wateris an acceptable diluent at low concentrations and can be dispersedrelatively easily in the mixture. Under these circumstances, water isused to reduce the viscosity (initially from thousands to hundreds ofthousands centipoise). Water-based capsules, such as those made from aprotein or polysaccharide material, for example, could be dispersed insuch a medium and coated, provided the viscosity could be sufficientlylowered. Curing in such systems is generally by ultraviolet radiation.

III. EXAMPLES Example 1

The following procedure describes gelatin/acacia microencapsulation foruse in electrophoretic displays of the present invention.

A. Preparation of Oil (Internal) Phase

To a IL flask is added 0.5 g of Oil Blue N (Aldrich, Milwaukee, Wis.),0.5 g of Sudan Red 7B (Aldrich), 417.25 g of Halogenated hydrocarbon Oil0.8 (Halogenated hydrocarbon Products Corp., River Edge, N.J.), and73.67 g of Isopar-G® (Exxon, Houston, Tex.). The mixture is stirred at60° C. for six hours and is then cooled to room temperature. 50.13 g ofthe resulting solution is placed in a 50 mL polypropylene centrifugetube, to which is added 1.8 g of titanium dioxide (TiO₂) (E. I. du Pontde Nemours and Company, Wilmington, Del.), 0.78 g of a 10% solution ofOLOA 1200 (Chevron, Somerset, N.J.), in Halogenated hydrocarbon Oil 0.8,and 0.15 g of Span 85 (Aldrich). This mixture is then sonicated for fiveminutes at power 9 in an Aquasonic Model 75D sonicator (VWR,Westchester, Pa.) at 30° C.

B. Preparation of Aqueous Phase

10.0 g of acacia (Aldrich) is dissolved in 100.0 g of water withstirring at room temperature for 30 minutes. The resulting mixture isdecanted into two 50 mL polypropylene centrifuge tubes and centrifugedat about 2000 rpm for 10 minutes to remove insoluble material. 66 g ofthe purified solution is then decanted into a 500 mL non-baffledjacketed reactor, and the solution is then heated to 40° C. A six-blade(vertical geometry) paddle agitator is then placed just beneath thesurface of the liquid. While agitating the solution at 200 rpm, 6 g ofgelatin (300 bloom, type A, Aldrich) is carefully added over about 20seconds in order to avoid lumps. Agitation is then reduced to 50 rpm toreduce foaming. The resulting solution is then stirred for 30 minutes.

C. Encapsulation

With agitation at 200 rpm, the oil phase, prepared as described above,is slowly poured over about 15 seconds into the aqueous phase, alsoprepared as described above. The resulting oil/water emulsion is allowedto emulsify for 20 minutes. To this emulsion is slowly added over about20 seconds 200 g of water that has been preheated to 40° C. The pH isthen reduced to 4.4 over five minutes with a 10% acetic acid solution(acetic acid from Aldrich). The pH is monitored using a pH meter thatwas previously calibrated with pH 7.0 and pH 4.0 buffer solutions. Stirfor 40 minutes. 150 g of water that has been preheated to 40° C. is thenadded, and the contents of the reactor are then cooled to 10° C. Whenthe solution temperature reaches 10° C., 3.0 mL of a 37% formalinsolution (Aldrich) is added, and the solution is further stirred foranother 60 minutes. 20 g of sodium carboxymethylcellulose (NaCMC) isadded, and the pH is then raised to 10.0 by the addition of a 20 wt %solution of sodium hydroxide (NaOH). The thermostat bath is then set to40° C. and allowed to stir for another 70 minutes. The slurry is allowedto cool to room temperature overnight with stirring. The resultingcapsule slurry is then ready to be sieved.

D. Formation of Display

Two procedures for preparing an electrophoretic display are from theabove capsule slurry are described below.

1. Procedure Using a Urethane Binder

The resulting capsule slurry from above is mixed with the aqueousurethane binder NeoRez R-9320 (Zeneca Resins, Wilmington, Mass.) at aratio of one part binder to 10 parts capsules. The resulting mixture isthen coated using a doctor blade onto a 0.7 mm thick sheet of indium-tinoxide sputtered polyester film. The blade gap of the doctor blade iscontrolled at 0.18 mm so as to lay down a single layer of capsules. Thecoated film is then dried in hot air (60° C.) for 30 minutes. Afterdrying, the dried film is hot laminated at 60° C. to a backplanecomprising a 3 mm thick sheet of polyester screen printed with thickfilm silver and dielectric inks with a pressure of 15 psi in a hot rolllaminate from Cheminstruments, Fairfield, Ohio. The backplane isconnected to the film using an anisotropic tape. The conductive areasform addressable areas of the resulting display.

2. Procedure Using a Urethane/Polyvinyl Alcohol Binder

The resulting capsule slurry from above is mixed with the aqueous bindercomprising a mixture of NeoRez R-966 (Zeneca Resins) and a 20% solutionof Airvol 203 (a polyvinyl alcohol, Airvol Industries, Allentown, Pa.)at a ratio of one part Airvol 203 solution to one part NeoRez R-966 tofive parts capsules. The resulting mixture is then coated using a doctorblade onto a 0.7 mm thick sheet of indium-tin oxide sputtered polyesterfilm. The blade gap of the doctor blade is controlled to 0.18 mm so asto lay down an single layer of capsules. The coated film is then driedin hot air (60° C.) for 30 minutes. After drying, a thick film silverink is then printed directly onto the back of the dried film and allowedto cure at 60° C. The conductive areas form the addressable areas of thedisplay.

Example 2

The following is an example of the preparation of microcapsules by insitu polymerization.

In a 500 mL non-baffled jacketed reactor is mixed 50 mL of a 10 wt %aqueous solution of ethylene co-maleic anhydride (Aldrich), 100 mLwater, 0.5 g resorcinol (Aldrich), and 5.0 g urea (Aldrich). The mixtureis stirred at 200 rpm and the pH adjusted to 3.5 with a 25 wt % NaOHsolution over a period of 1 minute. The pH is monitored using a pH meterthat was previously calibrated with pH 7.0 and pH 4.0 buffer solutions.To this is slowly added the oil phase, prepared as described above inEx. 1, and agitation is increased to 450 rpm to reduce the averageparticle size to less than 200 μm. 12.5 g of a 37 wt % aqueousformaldehyde solution is then added and the temperature raised to 55° C.The solution is heated at 55° C. for two hours.

Example 3

The following is an example of the preparation of microcapsules byinterfacial polymerization.

To 44 g of the oil phase, prepared as described above in Ex. 1, is added1.0 g of sebacoyl chloride (Aldrich). Three milliliters of the mixtureis then dispersed in 200 mL of water with stirring at 300 rpm at roomtemperature. To this dispersion is then added 2.5 mL of a 10 wt. %aqueous solution of 1,6-diaminohexane. Capsules form after about onehour.

Encapsulated electrophoretic displays and materials useful inconstructing them are therefore described. Additional aspects andadvantages of the invention are apparent upon consideration of theforegoing. Accordingly, the scope of the invention is limited only bythe scope of the appended claims.

1. An electrophoretic display comprising walls defining a cavitycontaining at least a refractive index matching fluid, the refractiveindex matching fluid moving within said cavity to create an opticallyhomogeneous cavity upon application of a first electric field.
 2. Thedisplay of claim 1, further comprising an internal phase.
 3. The displayof claim 2, wherein said internal phase is porous.
 4. The display ofclaim 3, wherein said internal phase is an alkylcellulose.
 5. Thedisplay of claim 4, wherein said alkylcellulose is selected from thegroup consisting of methylcellulose, methylhydroxyethylcellulose,hydroxyethylcellulose, hydroxypropylmethylcellulose,carboxymethylcellulose, and sodium carboxymethylcellulose.
 6. Anencapsulated electrophoretic display, comprising a capsule containing atleast two immiscible fluids, each fluid having a different refractiveindex such that the fluids create a first optical effect, wherein atleast one of the fluids within the capsule moves to create a secondoptical effect in response to an electric field.
 7. The display of claim6, wherein a planar index mismatch results from the motion of the fluidsin the capsule.
 8. The display of claim 6, wherein a non-planar indexmismatch results from the motion of the fluids in the capsule.
 9. Thedisplay of claim 6 wherein at least one of the fluids contains a dye.